The prior art is replete with methods for the manufacture of alumina-based catalysts comprising molybdenum and one or both of the Group VIII metals, cobalt and nickel. Such methods involve a great number of combinations of various impregnation, coprecipitation, comulling techniques and the like. In many of these procedures, the molybdenum component is introduced in the form of solid ammonium heptamolybdate, or ammoniacal solutions thereof. Ammoniacal impregnating solutions often also comprise ammonia complexes of salts of the Group VIII metal component, the nitrates usually being preferred. The presence of ammonia or ammonium salts in any of these procedures leads to expensive requirements for the containing of ammonia vapors, and for ultimately removing ammonium ions from the catalyst. The latter is usually effected by washing procedures, leading to aqueous waste disposal problems, or by calcination which generates air pollutants such as nitrogen oxides through combustion of some of the ammonium ions. Nitrogen oxides are also generated in the thermal decomposition of nitrate ions associated with the Group VIII metal component. The various washing procedures and/or multiple impregnation steps often lead to the necessity for multiple drying and/or calcination steps. Any of these undesirable consequences substantially increase catalsyt manufacturing costs.
One very simple procedure which has been employed in the art, and which avoids the foregoing problems, involves mulling together powdered MoO.sub.3 with powdered alumina and an oxide form of the Group VIII metal component. Such mixtures may be mulled either in the wet or dry state and subsequently formed into extrudates or pellets, and calcined. This procedure however does not provide an optimum combination, either physically or chemically, of all the catalyst components. It is generally believed that the preparation method should provide maximum interaction between the alumina and the molybdenum component, which comulling of the oxides in powdered form does not provide. Also, this method inherently results in very low surface areas of the active metal components. As a result catalysts prepared by the simple comulling of the oxide components always display very low activities.
I have now discovered a relatively simple preparation technique which substantially avoids the disadvantages, while retaining the principal advantages of the foregoing methods. This new procedure involves the following basic steps:
(1) reacting in an aqueous slurry at least one carbonate or hydroxide of nickel or cobalt with at least a stoichiometric proportion of MoO.sub.3 and/or molybdic acid to form finely divided crystalline cobalt and/or nickel molybdate in aqueous suspension;
(2) mixing the aqueous suspension with sufficient alumina hydrate consisting essentially of gelatinous boehmite to provide the desired proportion of Al.sub.2 O.sub.3 in the finished catalyst
(3) digesting and/or mulling the resulting mixture for at least about 0.5 hour with at least sufficient water to provide an extrudable plastic mixture;
(4) recovering from step (3) an extrudable plastic mixture;
(5) extruding the plastic mixture to provide extrudates of desired size and shape; and
(6) drying and calcining the extrudates.
It will be seen that the foregoing procedure requires no washing steps and but a single calcination step, does not employ ammonia, ammonium salts or nitrate salts, and generates no air or water pollutants. At the same time, in steps (3), (4), (5) and (6), an intimate interaction occurs between the boehmite alumina and the molybdate component, such that no cobalt and/or nickel molybdate is detectable by X-ray diffraction in the final catalyst. The activity of the resulting catalysts for desulfurization and denitrogenation is in most cases equal or superior to prior art catalysts of the same metals content prepared by conventional impregnation or coprecipitation methods.